An example of a conventional speaker is described in Japanese Patent Unexamined Publication No. H07-162992. FIG. 11 is a cross-sectional view of a conventional speaker showing a construction of the speaker.
In FIG. 11, the conventional speaker includes lower plate 20, magnet 21, upper plate 22, frame 24, diaphragm 25, edge 26, voice coil 27, dumper 28, and dust cap 29. Lower plate 20 is made of magnetic material, and pole 20a is formed at a center of lower plate 20. Magnet 21 is formed in a ring shape. A magnet for a speaker application is generally made of a material of neodymium group or of ferrite group, as magnet 21. Upper plate 22 is also formed in a ring shape and is made of magnetic material. Upper plate 22, magnet 21 and lower plate 20 constitute magnetic circuit 23. Magnetic gap 23a is formed in a ring shape between a peripheral wall of pole 20a of lower plate 20 and an inner peripheral wall of upper plate 22.
Frame 24 is bonded with upper plate 22 at a center of frame 24. Diaphragm 25 is bonded through edge 26 to a peripheral edge of frame 24 at a outer peripheral part of diaphragm 25, and an inner peripheral part of the diaphragm 25 is bonded with voice coil 27 placed in magnetic gap 23a in a freely movable manner.
Damper 28 is bonded with voice coil 27 at an inner peripheral part of damper 28, and an outer peripheral part of damper 28 is bonded with frame 24, thus supporting voice coil 27. Dust cap 29 is a dust-roofing cover attached to an upper center part of diaphragm 25.
Thus constituted conventional speaker generates an up and down amplitude movements of voice coil 27 with a sound signal inputted from an outside source (not illustrated) converted to a mechanical signal by the Fleming's left-hand rule, vibrating diaphragm 25 bonded with voice coil 27 and reproducing a sound.
In recent years, various electronic devices require that speakers using for the electronic devices reproduce a higher-quality sound. In-car speakers are especially required a light in weight with the high-quality sound. For this reason, many proposals are made for improving magnetic circuit 23 and diaphragm 25.
For example, diaphragm 25 is made lighter in weight by reducing a thickness of material or by using a material in a low density. However, diaphragm 25 which is made lighter in this way has lower modulus of elasticity, causing a split resonance of diaphragm 25, or causing a resonance of edge 26 bonded with the outer peripheral part of diaphragm 25. FIG. 12 exemplarily shows graphical data indicating a frequency characteristic of a conventional speaker. A unit of a vertical axis, which indicates a sound pressure characteristic 111, second-harmonic distortion characteristic 112 and third-harmonic distortion characteristic 113, is in dB SPL (Sound Pressure Level). A unit of a vertical axis, which indicates impedance characteristic 114, is in ohm. The characteristic of a sound pressure level versus frequency is sometimes called a SPL characteristic, hereinafter.
In order to control the split resonance, the inventors of this invention proposed a technique in Japanese Patent Application No. 2003-354832. The technical can control the split resonance of diaphragm 25, however, it is difficult to control the resonance of edge 26, so is hard to realize both of the requirement, light in weight and the high sound quality.